In devices such as batteries, which have a housing with an enclosed air volume that is subject to temperature changes and hence to changes in volume, a pressure equalization element (PEE) is normally used. This is a microporous membrane, e.g. one made of polytetrafluoroethylene, such as a Gore-Tex membrane, which is permeable to air but watertight. This PEE ensures complete pressure equalization, that is to say the same air pressure is established inside and outside the housing with a short delay. This pressure equalization is necessary if the housing cannot withstand the possible difference between the internal and the external pressure. A pressure difference of up to 1 bar is possible due to changes in temperature and external pressure during transportation by aircraft, for example.
One problem with the use of such pressure equalization elements with water-impermeable membranes, such as a Gore-Tex® membrane, is that, although they are impermeable to water in the liquid state of aggregation, they are not permeable to water vapor. If the external atmosphere is humid, moist air can penetrate the housing. If there is a region within the device which is cooled to temperatures below the external temperature, there may be condensation of this atmospheric moisture on the cooled regions. This condensation can lead to failures of the device.